I can't think of a good reason.
Perhaps someone thought it was a way to give an already privileged user
(other than the Ingres installation owner) privileged access to Ingres
too.
Personally I like to have a user called "ingres" to own the installation
and be in control of its processes and resources, and then I create
user IDs to own databases and hence to be the DBA of those databases,
with DBA privileges in them. I think that is the best way to
compartmentalize privilege in an Ingres environment.
Roy